Alkali solution treatment of cathodically chromated metal surface



United States Patent Oflice 3,518,169 Patented June 30, 1970 7 3,518,169 l ALKALI SOLUTION TREATMENT OF CATHOD- ICALLY CHROMATED METAL SURFACE v Taro Oyama and Tsuneo Inui,,Kudamatsu-shi, Japan, assignors to Toyo Kahan Co., Ltd., Tokyo, Japan, a corporation of Japan v No Drawing. Filed July 13, 1965, Ser. No. 471,703

Int. Cl. C23f 13/00; C2311 11/00 US. Cl. 204-35 6 Claims ABSTRACT on THE DISCLOSURE The present invention relatesto a process for improving the properties of the coating formedin the cathodic chromating of metal surfaces. Cathodic chromating is a process in which hexavalent chromium ionderived from chromic acid, chromates or dichromates in a treating solution is electrolytically reduced, mainly to the trivalent state, to form a protective coating of a hydrated chromium oxide complex on a suitable for the formation of a uniform hydrated chromium oxide coating, of blue or bluish purple color, which provides superior corrosion resistance and lacquer adhesion property.

If no sulfate anion is present in the chromating electrolyte, the film formed shows yellowish stains and has poor corrosion resistance and lacquer adhesion property. Thus, sulfate anion is'involved in the electrolytic reduction of hexavalent chromium. In that condition, however, it is physically adsorbed in the hydrated chromium oxide film on metal surfaces. The sulfate anion adsorbed in the film is water soluble and accelerates I the corrosion of treated metal surfaces under highly humid conditions. By heating the metals, this soluble sulfate anion changes to insoluble form in the film, but a number of cracks show in the film, so that corsion resistance is not improved by simple heating.

As described above, the addition agent which gives sulfate anion is necessary to form a superior film but it gives a detrimental effect on the corrosion resistance of the formed film. Such a behavior as sulfate anion may be cathode metal. In the present specification the expression hydrated chromium oxide coating is employed, for the film formed by chromating. An analysis of the formed film shows that it contains no other metals than chromium and does not consist: of simple metallic chromium. The analysis also shows the film contains a very little amount of an anion which originallypresents in the chromating solution as an addition agent. Y

When the wetted hydrated chromium oxide coating formed immediately after the chromating is dried at an appropriate temperature, the anion in the coating is fixed as a member of a co-ordinated complex structure of the coating. In a wetted stage, however, the anion is not firmly fixeid in the structure and behaves as if it absorbs physical y.

As shown in the specific examples in the present specification, the conditions for a cathodic chromating process are quite different from those for chromium plating, dip chromating and anodic chromating. A coating formed in chromium plating is a simple metallic chromium and the one formed in dip chromating is mainly a chromium chromate and in some cases is a mixture of a chromate and an oxide of metal to be coated, while the one formed in anodic chromating is a mixture of a chromate and an oxide of metal to be coated.

In the cathodic chromating process various addition agents such as boric acid, phosphoric acid, sulfuric acid, disulfonic acid, nitric acid, acetic acid, or their respective salts and halogen compounds are added to the electrolyte which mainly contains hexavalent chromium ion derived from chromic acid, chromates or dichromates as described in US. Pats. Nos. 2,733,199, 2,780,592, 2,769,774, 2,998,- 361, 3,032,487, 3,257,295, 3,288,691 and 3,296,100.

Of these addition agents, disulfonic acid, sulfuric acid or their respective salts which gives sulfate anion is most observed in strongly acidic anions :such as nitrate, chlorate, perchlorate, and halogen anions in cathodic chromating In order to remove the anion adsorbed in the hydrated chromium oxide film and to improve the properties of the film, an immersion or a cathodic treatment, immediately after the previous cathodic treatment and before the surface of the metal is allowed to dry, in an aqueous solu tion containing at least one alkaline salt selected from the group consisting of ammonium hydroxide, alkali metal hydroxides, alkali earth metal hydroxides and alkaline salts of weak acids before drying the film formed in cathodic chromating is most efiective.

- An alkaline salt used in the present invention is selected from the following salts.

(l) Ammonium hydroxide. (2) Alkali metal hydroxides: LiOH, NaOl-I, KOH, RbOH, CsOH.

(3) Alkali earth metal hydroxides: Ba(0H) -8-H O, Sr( OH 2 8'H O. (4) Alkaline salts of weak acids: Acetates, CH COOLi-2H O, CH C0ONa-3H O,

CH COOK, CH3CO0NH4 Borates, Li B O7'5H2O, -Na2B407 101 120, K B O -5H 0,

NH4'HB407 Carbonates and bicarbonates, Li CO Na CO l0H O, I K CO -3/2'H O, Rbzcog, CS2CO3, (NH4)2CO3'H20,

. NaKCO -6H O, NaI-ICO KHCO NH HCO Chromates and dichromates, Li CrOl -2H O,

HCOONH;

The conditions required for performing the process described in the present invention will be summarized as follows:

(1) Concentration of alkaline salts-l-lOO g./ liter (2) Treating temperature3070 C.

(3) Treating time (in the case of immersion)130 secends (4) Treating time (in the case of cathodic treatment)- not more than 2 seconds (5) Treating current density (only in the case of cathodic treatment)-0-10 a./sq. dm.

The optimum concentration of alkaline salts is within the range from 1 g./liter to 100 g./liter. Within this range, one or more than one alkaline salts can be added. Generally an increase in concentration results in a higher removal efficiency of adsorbed anion. However, when the concentration of alkaline salts exceeds 100 g./liter, dissolution of the film formed in cathodic chromating begins to occur. When the concentration is less than 1 g./liter, the process needs a longer-period to remove adsorbed anion, which is impractical in high speed commercial operation.

Of the conditions, an increase in temperature results in a higher removal efficiency of adsorbed anion. At higher temperatures, however, the evaporation loss of water increases rapidly. Accordingly, operation should preferably be controlled at a temperature below 70 C. On the other hand, a decrease in temperature results in a lower removal efficiency of adsorbed anion. When the temperature is below 30 C., the process requires a longer period. Therefore, it is preferable to conduct the operation within the temperature range between 30 C. and 70 C.

In an immersion treatment, an increase in immersion time results in a higher removal efiiciency of adsorbed anion, but with a concentration of l-100 g./liter of salt and at a temperature of 30-70 C., the immersion time of 1-30 seconds is sufficient. When the period exceeds 30 seconds, dissolution of the formed film is initiated.

In the high speed operation which requires continuous and short-time treatment, the period of, e.g., seconds, is often not practical. When the treating period of not more than 2 seconds are required, a cathodic treatment is necessary in the alkaline solution mentioned above because the immersion treatment is less effective. In this case, the current density should be controlled to within 10 a./ sq. dm. If the current density exceeds 10 a./ sq. dm., the formed film will be partially dissolved and the corrosion resistance of the film will not be improved.

An immersion treatment in hot water of 80-90 C. is also effective to remove adsorbed anion but this takes as long as 2 minutes which is not practical in high speed commercial operation.

When the film formed in cathodic chromating is dry, an immersion treatment in an alkaline solution is less effective for the removal of adsorbed anion, but in such case, the dried film behaves as a hard barrier against the action of an alkaline solution. Under ordinary conditions, i.e., in concentrations of alkaline salt, in the period of immersion, in current density, temperature, etc., as heretofore disclosed, the action of the alkaline solution is delayed. Therefore in such a very short-period operation as in the commercial production, a cathodic treatment in a concentration of more than 100 g./liter of alkaline salts and at a temperature of more than 70 C. is necessary to obtain the effective action of alkali.

On the other hand in the wetted film before drying, the action of alkali is always effective in lower concentrations at lower temperatures even in high speed operation as described above. Therefore, the alkali treatment must be immediately performed before allowing the film formed in cathodic chromating to be dried.

After cathodic chromating, iron and steel treated according to the present invention can withstand outdoor exposure tests for 2 months in winter and for 15 days in summer and resist salt spray tests for 48 hours without any sign of rust. They further resist humidity chamber tests under the condition of relative humidity of 90%, at 40 C., for one week without any sign of discoloration of the film.

After applying a film of about 10 microns of modified alkyd or modified epoxy lacquer on steel sheet treated according to the present invention, the lacquered sheet was deeply drawn to the cup with a drawing ratio of 2.2 and stretch formed in the cup wall. The lacquer adhesion on the stretched part of the cup side quite withstood even the adhesive tape test. Furthermore, no adhesion loss of the lacquer was found on the cup wall, even when tested with adhesive tape after the cup was immersed in a boiling 3% sodium chloride solution for one hour.

Metals having the following surface are suitable for treatment according to the present invention; iron, steel, aluminum, zinc, tin, nickel, chromium, their respective alloys.

The examples of the present invention are as follows:

EXAMPLE 1 A 0.25 mm. cold rolled low carbon steel sheet, socalled black plate, is cathodically cleaned for 20 seconds at a current density of 4 a./sq. dm. at C. in a 7% sodium hydroxide solution, then rinsed with water, pickled for 10 seconds at room temperature in a 7% sulfuric acid, again rinsed with water, and immediately treated cathodically using a lead antimony alloy anode under the following condition in an electrolyte of the following composition, again immediately rinsed with water, then immediately dipped in the treating solution of the present invention indicated below, and again immediately rinsed with water, and dried.

1) The electrolyte and the condition of cathodic chromating:

Chromic acid-50 g./ liter Ethyl alcohol--0.2 g./liter Sulfuric acid-0.2 g./1iter Hydrofiuosilicic acid0.3 g./liter Temperature-50 C.

Current density-20 a./ sq. dm. Time-10 seconds (2) The treating condition of the present invention:

Sodium carbonate-100 g./liter Temperature55 C. Dipping time-2 seconds A transparent, bluish purple film was formed. Steel sheet thus treated showed very little rusty spots when subjected to a salt spray test with a 5% sodium chloride solution for 48 hours at 35 C. It also showed no rusty spot and no discoloration when subjected to a humidity chamber test with a relative humidity of for one week at 40 C.

EXAMPLE 2 A 0.5 mm. cold rolled low carbon steel sheet was subjected to the same pretreatment and the same cathodic chromating as described in Example 1, immediately rinsed with Water, then immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with Water, and dried.

Ammonia water (28% )30 g./liter Temperature40 C. Dipping time10 seconds A transparent, bluish purple film Was formed. Steel sheet thus treated showed no rusty spot when subjected to the same salt spray test as in Example 1 for 24 hours. Furthermore, when the steel sheet thus treated was coated with about 10 microns of modified alkyd' white enamel and EXAMPLE 3 r The same kindjpf cold rolled low carbon steel sheet Was subjected to the same pretreatment as described in Example 1, immediately treated cathodically using a lead antimony alloy anode under the following condition in an electrolyte of the following composition, immediately rinsed with water, then' immediately dipped in the treating solution of ,the present inventionunder the following condition and again immediately rinsed water, and 'dried."

1) The electrolyte and the condition of cathodic cromating:

Chromic-aeid-SO g./ liter Ethyl alcohol-+0.25 g./ liter Sulfuricaeid- -QSO. g./ liter 1 1 Temperature-50 C. Current density-20 a./ sq. dm. Time-20 seconds .i:

(2) The treating condition bf the present invention: Ammonium carbonate--'-1' g'.7liter Temperature'-50;C M Dip ingtime -ZJOsecondsi' I, H U A transparent, bluish yellowfi-lm was formed. Steel sheet thus-treated had-the similar corrosion resistance to those in Example 1 and-the similar lacquer adhesion property to thosein Example. 2. i 1 i "EX A M PIIE 4 The same kind of cold rolled carbon steel sheet was subjected to the same pretreatmentas described in Example 1, treated inthe same cathodic chromating as described in Example3,- immediately rinsed with water, then immediately treated cathodically in the treating solution of the present invention under the following condition and again immediately rinsed with water, and dried.

Trisodium phosphate70 g./ liter Temperature60 C.

Current density-40 a./ sq. dm. Treating time-1 second A bluish yellow, transparent film was formed. Steel sheet thus treated was found to have almost no rusty spots when subjected to the same salt spray test as in Example 1 for 48 hours and had the similar lacquer adhesion property to those in Example 2, and when the deeply drawn cup of the lacquered sheet was immersed in a boiling 3% sodium chloride solution for one hour, the adhesive tape test failed to peel oif the lacquer film.

EXAMPLE 5 The same kind of cold rolled low carbon steel sheet was subjected to the same pretreatment as described in Example 1, and then immediately treated cathodically using a lead antimony alloy anode under the following condition in an electrolyte of the following composition, again immediately rinsed with water, then immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water, and dried.

1) The electrolyte and the condition of cathodic chromating:

Chromic acid-60 g./liter Phenol-2.4-disulfonic acid-0.6 g./ liter Temperature-50" C.

Current density20 a./ sq. dm. Time-30 seconds 6 (2) The treating condition of the present invention:

Potassium carbonate-50 g./ liter Potassium dichromate--10 g./liter Temperature-70 C.

Dipping time-2 seconds A blue, transparent film was formed. Steel sheet thus treated showed a few rusty spotswhen subjected to the outdoor exposure test during 2 months. Furthermore, when the steel sheet treated was coated with about 10 microns of melamine modified alkyd enamel and subjected to the same salt spray test as in Example 1 for hours, no blister of the enamel was found.

EXAMPLE 6 The same kind of cold rolled low carbon steel sheet was subjected to the same pretreatment as described in Example 1, immediately treated in the same cathodic chromating as described in Example 5, immediately rinsed with water then immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water, and dried.

Lithium carbonate-30 g./ liter Temperature-60" C. Dipping time-5 seconds A blue, transparent film was formed. Steel sheet thus treated showed no rusty spot after being subjected to a humidity chamber test as in Example 1. This steel sheet was coated with about 10 microns of modified alkyd white enamel and deeply drawn to the cup with drawing ratio of 2.2, no adhesion loss of the enamel was foun on the cup side. v

EXAMPLE 7 The same kind of cold rolled low carbon steel sheet was subjected to the same pretreatment and the same cathodic chromating as described in Example 1-, then immediatelyrinsed, with water, immediately dipped. in the treating solution of the present invention under the following condition and again immediately rinsed with water, and dried.

Barium hydroxide-20 g./liter Temperature70 C. Dipping time5 seconds A bluish purple film was formed. Steel sheet thus treated had the similar properties to those in Example 1.

EXAMPLE 8 The same kind of cold rolled low carbon steel sheet was subjected to the same pretreatment as described in Example 1, immediately treated cathodically using a lead antimony alloy anode under the following condition in an electrolyte of the following composition, again immediately rinsed with water, then immediately dipped in the treatment solution of the present invention under the following condition and immediately rinsed with water, and dried.

1) The electrolyte and the condition of cathodic chromating:

Sodium dichromate-SOO g./ liter Chromium sulfate-10 g./liter Ammonium sulfate-20 g./ liter Temperature50 C.

Current density-25 a./sq. dm. Time8 seconds (2) The treating condition of the present invention:

Ammonia water (28% )--40 g./ liter Temperature-601 C. Dipping time-10 seconds A blue, transparent films was formed. Steel sheet thus 7 treated had the similar corrosion resistance to' those in Example 1.

EXAMPLE 9 A 0.5 mm. aluminum sheet (99.0%) was cleaned for 10 seconds at room temperature in 4% sodium dicarbonate solution, rinsed with water, immediately treated cathodically using a lead antimony alloy anode under the following condition in an alectrolyte of the following composition, again immediately rinsed with water, then immediately dipped in the treating solution of the present invention and again immediately rinsed with water, and dried.

(1) The electrolyte and the condition of cathodic chromating:

Chromic acid50 g./1iter Ethyl alcohol0.15 g./ liter Sulfuric acid0.20 g./liter Ammonium fiuosi1icate1.0 g./liter Temperature-40 C.

Current density-12.5 a./sq. dm. Time20 seconds (2) The treating condition of the present invention:

Ammonium carbonate-30 g./liter Temperature55 C. Dipping time-10 seconds A grayish white film was formed. Aluminum sheet thus treated did not change when subjected to a salt spray test as in Example 1 for 120 hours.

What we claim is:

' 1. A process for treating metal which comprises cathodically chromating said metal in the presence of sulfate ions, rinsing the chromated metal with water before being allowed to dry, removing entrapped sulfate ions by immersion in a aqueous solution containing a total of from 1 to 100 grams per liter of at least one water-soluble alkaline compound selected from the group consisting of ammonium hydroxide, alkali metal hydroxides, alkaline earth metal hydroxides, and alkali metaland ammonium acetates, borates, carbonates, bicarbonates, chromates, dichromates, formates, oxalates, phosphates, pyrophosphates, hypophosphates, phosphites, hypophosphites and silicates applying a cathodic current to the catholically chromated surface wh le immersed in said aqueous solution, removing from bath, rinsing with water, and thereafter drying.

. 2. A process according to claim 1 wherein the chromated metal is immersed for from 1 to 30 seconds.

3. A process according to claim 1 wherein the cathodic treatment is conducted for not more than 2 seconds.

4. A process according to claim 1 wherein the temperature of the aqueous solution is from 30 to C. 5. A process according to claim 1 wherein the current density for cathodically treating the metal is not more than 10 amperes per square decimeter.

6. A process according to claim 1 wherein the metal to be treated has a surface selected from the group consisting of iron, steel, aluminum, zinc, tin, nickel, chromium and alloys thereof.

References Cited UNITED STATES PATENTS W. B. VANSISE, Assistant Examiner U.S. Cl. X.R. 2014-5 6 

